The Slope Stability Radar (SSR) is a ground based interferometric radar, designed to detect the precursor movements to slope failure in open cut mines. It achieves this by remotely monitoring the movement of the rock face, and using these movements to predict slope failure. The monitored region is scanned regularly, producing a close to real time assessment of the rock face and allows mine staff to react quickly to changes in the rock face. To achieve maximum safety benefit high accuracy and reliable measurements are required.
A mine using the SSR is reliant upon the measurement data from the system for safety warnings. Any disturbance to the signal could, unless identified, be incorrectly interpreted as a wall movement. Haul trucks and other mining vehicles can block the path between the SSR and target section of wall. Mine plant equipment, such as pumps and lighting plants may be parked in regions of the image. Grass and other vegetation may also be growing on the face in the field of view of the radar. Even the air itself between the SSR and the wall can appear to induce movements, when there is a change in atmospheric pressure, humidity or temperature. This affects the refractive index of the air and thus the speed of the radar wave through the medium.
All these effects reduce the precision of the measurements. The regional disturbances of the haul trucks often disguise the true movements in those areas. Vegetation causes random fluctuations in the signal, which could be interpreted as movement. If the reduced measurement precision of some areas is left unidentified, the user's confidence for other more stable areas on the wall will be reduced. Finally atmospheric changes appear to produce global movements of the whole wall which, if left uncompensated, would appear as fluctuating movements of the wall, and not allow the true movements to be detected. An even worse phenomenon for atmospherics is that they can also produce a permanent change to the measured displacement due to signal ambiguity issues resulting from the measurement method. The result of these effects can be a lack of trust by the user in the measurements made by the SSR. To regain the trust of the user these effects need to be corrected or removed, or at least identified and displayed to the user.
To allow understanding and identification of these disturbances it is important to understand the method of operation of the SSR. This has been described previously in our granted U.S. Pat. No. 6,850,183. The SSR uses the phase of the returned signal to determine the movement of a wall slope. As explained in Reeves B. A. et al., “Developments in Monitoring Mine Slope Stability using Radar Interferometry”; IEEE Publication 0-7803-6359-0/00, pp. 2325-2327 and Bamler R. et al., “Synthetic aperture radar interferometry”, Inverse Problems. Vol. 14, pp 1-54 1998, phase change can be converted to displacement using the following formula:Δd=ΔØλ/4π+nλ/2  [1]
where Δd is the displacement, ΔØ is the measured change in phase, λ is the wavelength of the carrier frequency of the radar (32 mm) and n is an integer unknown.
The parameter “n” corresponds to the number of wavelength cycles the target has moved between scans. For small time intervals this is assumed to be 0. As the phase is a number between +/−π, the result is that for the measured distance change to be correct the actual distance change needs to be less than +/−λ/4 or +/−8 mm. As a result, processing methods need to be both robust for removal of disturbances to the phase change (and thus displacement) measurement, as well as being able to improve the accuracy of the estimation of the unknown “n” within the displacement calculation formula.
One of the processing techniques described in U.S. Pat. No. 6,850,183 to improve signal quality was atmospheric correction. In this technique, a single reference section of wall was used to determine the atmospheric effect on the signal. This calculated correction was then applied to the remainder of the wall. Simple processing based on changes in amplitude and phase for detection of vegetation and other spurious signals, such as trucks, was also discussed.
The problems with the known technique for disturbance rejection include:                Large atmospheric changes in the environment may induce a wrapping error when the difference between subsequent measurements is greater than +/−λ/4 mm;        As the range to a single atmospheric correction region increases the chance of getting a wrapping error increases, thus limiting the range of the system;        A wall movement in the atmospheric correction region will produce a false apparent movement of the wall;        A truck, vegetation or other type of disturbance in the atmospheric region will produce a false apparent movement of the wall;        The single atmospheric region correction works best for targets at a similar range. If the targets of interest are distributed over a large band of ranges, the atmospheric correction does not work effectively;        The known approaches do not compensate for temperature changes or other effects on the radar electronics;        Disturbing signals from trucks and other mining vehicles can cause step changes in the displacement measurements, confusing the user and making automatic alarming difficult; and        Vegetation on the rock face reduces the accuracy of the displacement measurements for that location, confusing the user and reducing confidence in the rest of the measurements.        
Identifying and rejecting these disturbances will significantly improve the precision of the SSR, increase confidence in the measurement accuracy, and reduce the number of false positive alarms. This will in turn improve the acceptance of the SSR technology leading to improved mine safety.